Re: [PATCH v2] usb: Un-deprecate -usbdevice (except for -usbdevice audio which gets removed)

[Markus Armbruster <armbru@redhat.com>]

Thomas Huth <thuth@redhat.com> writes:

> When trying to remove the -usbdevice option, there were complaints that
> "-usbdevice braille" is still a very useful shortcut for some people.

Pointer?  I missed it.

> Thus we never remove this option. Since it's not such a big burden to
> keep it around, and it's also convenient in the sense that you don't
> have to worry to enable a host controller explicitly with this option,
> we should remove it from he deprecation list again, and rather properly
> document the possible device for this option instead.
>
> However, there is one exception: "-usbdevice audio" should go away, since
> audio devices without "audiodev=..." parameter are also on the deprecation
> list and you cannot use "-usbdevice audio" with "audiodev".
>
> Signed-off-by: Thomas Huth <thuth@redhat.com>

To be frank, I don't like this.  At all.

-usbdevice comes with its own ad hoc mini-language, parsed by
usbdevice_create().  Syntax is DRIVER[:PARAMS], where PARAMS defaults to
"" and is parsed by an optional DRIVER-specific LegacyUSBFactory.

We already dropped multiple drivers: "host", "serial", "disk", "net"
(commit 99761176e, v2.12), and "bt" (commit 43d68d0a9, v5.0).

We've kept "audio" (dropped in this patch), "tablet", "mouse",
"keyboard", "braille", "ccid", and "wacom-tablet".  Only "mouse",
"tablet", "braille" are documented (fixed in this patch).

One more has crept in: "u2f-key" (commit bb014a810, v5.2).  It's buggy:

    $ qemu-system-x86_64 -S -usbdevice u2f-key
    qemu-system-x86_64: -usbdevice u2f-key: '-usbdevice' is deprecated, please use '-device usb-...' instead
    **
    ERROR:../qom/object.c:508:object_initialize_with_type: assertion failed: (type->abstract == false)
    Bail out! ERROR:../qom/object.c:508:object_initialize_with_type: assertion failed: (type->abstract == false)
    Aborted (core dumped)

Broken right in the commit that added the stuff.  The sugar never
worked, and should be taken out again.

Without a factory, "-usbdevice BAR" is sugar for

    -device BAZ -machine usb=on

"braille" is the only driver with a factory.  "-usbdevice braille" is
sugar for

  -device usb-braille,chardev=braille -chardev braille,id=braille
  -machine usb=on

It's buggy:

    $ qemu-system-x86_64 -S -usbdevice braille
    qemu-system-x86_64: -usbdevice braille: '-usbdevice' is deprecated, please use '-device usb-...' instead
[three seconds tick by...]
    Segmentation fault (core dumped)

It neglects to actually parse PARAMS:

    $ qemu-system-x86_64 -S -usbdevice braille:"I'm a Little Teapot"
    qemu-system-x86_64: -usbdevice braille:I'm a Little Teapot: '-usbdevice' is deprecated, please use '-device usb-...' instead
[three seconds tick by...]
    Segmentation fault (core dumped)

The whole machinery in support of optional PARAMS has long become
useless.

I fail to see why we could drop the sugar for serial, disk, net and host
devices, but not for the others.

The only one that has something approaching a leg to stand on is
braille.  Still, I fail to see why having to specify a backend is fine
for any number of other devices, but not for braille.

Does QEMU really need more ways to do the same things?  Underdocumented
and undertested ways.

Let's drop -usbdevice as planned.

If users need more time, we can extend the grace period.